The researchers examined the impacts of smoke - resulting from spring agricultural land-clearing fires in Central America - on a severe weather outbreak that occurred during the afternoon and evening of April 27, 2011.

The event produced 122 tornadoes, resulted in 313 deaths across the south-eastern United States, and is considered the most severe event of its kind since 1950.

"These results are of great importance, as it is the first study to show smoke influence on tornado severity in a real case scenario," said co-lead author Gregory Carmichael, professor at the University of Iowa in the US.

Smoke lowered the base of the clouds and increased wind shear, defined as wind speed variations with respect to altitude. Together, these two conditions increased the likelihood of more severe tornadoes, the researchers added.

"Severe weather prediction centres do not include atmospheric particles and their effects in their models, and we show that they should at least consider it," Carmichael said.